The present invention relates generally to optical time domain reflectometry and more specifically to an optical time domain reflectometer using a step-impulse response for characterizing an optical fiber.
A traditional optical time domain reflectometer (OTDR) tests an optical fiber by launching optical pulses at a particular wavelength into the fiber under test and acquiring waveform data to represent a display trace of the return optical energy from the fiber. The OTDR, under operator control, determines the portion of the test fiber to be examined, the pulse width and wavelength of the optical pulses to be launched into the test fiber, the sample density or spacing between acquired data points, the amount of averaging for each acquired data point, and the like. A series of optical pulses are launched into the test fiber. During the period between each test pulse, a return reflected optical signal in the form of Rayleigh scattering and reflections due to mechanical spices, connector, and the like is received in an optical receiver, converted to an electrical signal and sampled in accordance with the preselected sample density. The acquired waveform data is stored in memory and further processed to locate and measure events on the test fiber. When using this technique, a trade off must be made between event resolution and dynamic range.
In "Optical Time-Domain Reflectometry" by Duwayne Anderson and Florian Bell, published by Tektronix, Inc., 1997, a number of OTDR parameters are discussed. One important parameter for an OTDR is its ability to resolve closely spaced events. This relates to the instrument's time resolution and the ability of the OTDR to identify two discrete reflective events separated by a short distance, called event resolution or event dead zone (EDZ). In a single mode OTDR this value can be as low as one meter. For a multimode OTDR the shortest EDZ is about twenty centimeters.
A simplified OTDR system response can be modeled as a single-pole amplifier with the rising edge of a pulse given by: EQU 5 log(1-e.sup.-t/.tau.) (1)
where t is the time constant of the resistive-capacitive (RC) network in the single-pole model. The rate at which the pulse falls is given by: ##EQU1##
Because the system has a limited bandwidth, the fall time of the pulse is not infinitely fast. If there are two reflective events closely spaced, there is a critical separation such that the signal from the first event does not significantly fall before the signal from the second event becomes appreciable. When two refections are spaced closer together than this limit, they become essentially indistinguishable. This is defined as the event dead zone. It is equal to the distance between the leading edge of a reflection and the point on the falling edge where the signal level drops 3 dB below the top of the refection. The EDZ is defined in terns of how a human operator would identify two closely spaced events. However, some modern OTDRs with sophisticate event-detection algorithms are able to automatically detect events whose spacing is smaller than the event dead zone.
Another parameter used for specifying OTDR performance is the attenuation dead zone (ADZ) or loss-measurement dead zone (LMDZ). Discrete reflective events in a fiber under test produce large return reflections that temporarily saturate the optical detector and preamplifier in the receiver. It can take as long as a microsecond (100 meters in distance, as viewed on and OTDR display) for the detector to recover fully from these reflective pulses. Even without saturation, the amplifier is still bandwidth limited so the receiver cannot return immediately to the backscatter level. An additional spurious signal, called detector tail, caused by slow currents in the detector may also be present. When any of these effects (saturation, bandwidth limitations, or tail) are present in sufficient quantities, no useful information can be easily obtained from the OTDR signal due to distortion. The distance over which the normal OTDR signal is distorted due to saturation, bandwidth effects, or detector tail, is the LMDZ or ADZ of the instrument.
U.S. Pat. No. 5,528,356, assigned to the assignee of the present invention, describes an OTDR that acquires and stores waveform data points having multiple waveform segments with each waveform segment having data points acquired using different pulsewidths, sample spacing and starting distance. The waveform segments are defined in terms of the noise floor. The gain of the OTDR receiver amplifier may be increased for the various waveform segment acquisitions in conjunction with other parameters, such as the pulsewidth, averaging and the like, to increase the signal to noise ratio within the segment. The width of displayed events on an OTDR are at a minimum equal to the pulsewidth of the interrogating pulses. For detecting closely spaced events, it is desirable to use a narrow pulsewidth interrogating pulse. However, the narrow pulsewidth interrogating pulses inject less optical energy into the fiber resulting in less dynamic range. Longer pulsewidths are used where dynamic range or signal-to-noise is more important. In the OTDR described in the '356 patent, narrow pulsewidth interrogating pulses are used for the first segment of the fiber, with increasing longer pulsewidth interrogating pulses used for subsequent segments. The use of narrow pulsewidth interrogating pulses over the first segment of the fiber is well suited for measuring closely spaced connectors and spices found in telephone company central offices. The longer pulsewidth interrogating pulses are well suited for measuring the fiber between central offices. However, there may be closely spaced connectors and splices at some distance from the central office that may not be detectable as separate events because of the pulsewidth of the interrogating pulse and the event dead zone and attenuation dead zone. In addition, even close-in closely spaced events may not be detectable as separate events using narrow pulsewidth interrogating pulses because of the bandwidth limits of the OTDR system.
What is needed is an optical time domain reflectometer having improved event detection resolution by compensating for the bandwidth and pulsewidth limitations of the OTDR.